Writing loops inside other loops
1. Nesting loops
2. Counting how many times the loop body will run

Writing loops inside other loops

Writing loops inside other loops is called nesting.

Nesting loops

An inner loop is nested inside the body of an outer loop. Each time the outer loop body runs an iteration, the entire inner loop runs.

Inner loops can access variables that are inside the outer loops they are nested in.

Counting how many times the loop body will run

Consider this example of nested loops:

Code

# 3x3 multiplication table
for a in range(1, 4):
    # Body of loop a
    for b in range(1, 4):
        # Body of loop b
        product = a * b
        print(str(a) + ' x ' + str(b) + ' = ' + str(product))

Output

x 1 = 1
x 2 = 2
x 3 = 3
x 1 = 2
x 2 = 4
x 3 = 6
x 1 = 3
x 2 = 6
x 3 = 9

The body of loop a, by itself, will run 3 times
The body of loop b, by itself, will run 3 times
Since loop b is nested inside loop a, loop b will run 3 times
So, the body of loop b will end up running 3 x 3 = 9 times in total

Suppose we change the range of loop b to range(1, 11):

The body of loop b, by itself, will run 10 times
So, the body of loop b will end up running 3 x 10 = 30 times in total

You can triple-nest loops. You can quadruple-nest loops. You can nest as many times as you need to.

Code

for x in range(3):
    for y in range(10):
        for u in range(4):
            for v in range(7):
                print(x, y, u, v)

The body of loop x will run 3 times in total
The body of loop y will run 3 x 10 = 30 times in total
The body of loop u will run 3 x 10 x 4 = 120 times in total
The body of loop v will run 3 x 10 x 4 x 7 = 840 times in total

You can also nest while loops in any way you like. But with while loops, you may not be able to count exactly how many times nested loops will run.